The present invention is directed to novel compositions useful for developing electrostatic images in electrophotography.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner". The toner is normally attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image can then be transferred to a support surface such as paper. The transferred image can subsequently be permanently affixed to the support surface as by heat.
Two types of development systems, dual component and monocomponent systems, are used for developing an electrostatic image. For example, in the "magnetic brush" process, a two component developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carrier particles in a brush-like configuration. This "magnetic brush" is engaged with an electrostatic-image bearing surface and the toner particles are drawn from the brush to the electrostatic image by electrostatic attraction. See, for example, U.S. Pat. No. 2,874,063.
In magnetic-brush development of electrostatic latent images, the developer is commonly a triboelectric mixture of finely-divided toner powder comprised of dyed or pigmented thermoplastic resin mixed with coarser carrier particles of a soft magnetic material such as ground, chemically reduced iron oxide particles, magnetite or the like. The conductivity of the ferromagnetic carrier particles which form the "bristles" of a magnetic brush provides the effect of a development electrode having a very close spacing to the surface of the electrophotographic element being developed.
While ordinarily capable of producing good quality images, conventional developing materials in dual component systems suffer deficiencies in certain areas. For example, the toner particles, which can be very small, on the order of 10 microns, tend to scatter throughout the copying machine. This can adversely affect the operation of the machine and can result in powder clouding on the paper. Further, some of the toner particles do not get an electrical charge and end up on the paper as a dirty background.
Attempts have been made to solve this problem by slightly magnetizing the toner such as including magnetite in the toner so that toner particles that are not drawn from the magnetic brush by electrostatic attraction remain on the magnetic brush due to magnetic attraction.
However, attempts to solve this problem by including magnetite or other magnetic material in the toners has created a new set of problems. One problem is that the bulk density of the toners is increased, thereby increasing the cost to the customer in that less copying per pound of toner can be effected. Further, the higher density of the toner can result in attrition of the carrier particles and "toner impaction", two phenomena discussed below. Further, only relatively low amounts of magnetite can be included in the toner, or else the toner will not come off the magnetic brush. It is difficult to uniformly disperse magnetite in low quantities in toner particles.
Difficulties are also experienced with the carrier particles. For example, with existing components, it is difficult to design a carrier particle with specific electrical and magnetic properties to be useful for a specific copying machine. Further, the use of metal in the carrier products makes manufacture difficult, the metal being difficult to grind and compound.
Other problems with existing carriers result from chipping and other degradation and attrition of the carrier particles. These small particles can end up on the paper, resulting in a poor copy.
Further, as the carrier particles are recycled through many cycles, many collisions occur between carrier particles and other surfaces in the machine. As a result of these collisions, toner particles carried on the surface of the carrier particles tend to become welded or otherwise forced onto the carrier surfaces, a phenomenon known as toner impaction. This can result in a change of the triboelectric value of the carrier and can contribute to the degradation of copy quality by eventual destruction of the toner carrying capacity of the carrier. This problem is especially aggravated with carriers prepared from high density materials such as iron, magnetite, or steel.
Attempts have been made to overcome some of these problems using a single component magnetic toner rather than a dual component system. An advantage of a single component system is that carrier particles are not needed. However, difficulty has been experienced in making uniform toner particles in that they need to contain at least a binder, a magnetic material, and pigment. Forming uniform particles with these three ingredients can be difficult.
Further, the magnetic materials used, such as zinc-doped magnetite, can be expensive, contributing to the higher cost of monocomponent developing systems compared to two component developing systems.
Further, the magnetic material increases the bulk density of the toner particles, since ferrites have a bulk density of from 2.2 to 2.7 grams/cc and steel has a bulk density of from 2.3 to 3.5 grams/cc. This high bulk density of the toner particles results in a "heavy" magnetic brush. This can cause imperfections in the developed image as the brush scours away toner particles from the electrostatic image.
Another problem experienced with monocomponent developing systems is difficulty in formulating a particular toner. It is difficult to obtain a satisfactory balance of the magnetic, electrical, color, and physical properties of the toner particles while maintaining an adequate level of resin so that the toner particles fuse when heated to make good quality copies. For example, when using magnetite, it may in some instances be very difficult to achieve the desired magnetic moment for the toner without so overloading the toner with magnetite that the toner particles cannot be easily fused to fix the image.
In view of these problems, it is apparent that there is a need for improved developers for developing electrostatic images.